The invention relates to a mixer/oscillator circuit comprising a mixer stage with an oscillator stage cconnected thereto.
When coupling an oscillator to a controlled mixer stage, the oscillator frequency may be affected. This is because a change in the control voltage causes changes in the current in transistors in the mixer stage. The impedance of the base emitter path of the transistor however depends on currents and thus changes with the collector current or the emitter current. The changes in impedance at the base electrode of the mixing stage transistors brought about by changes in the control voltage and at the same time forming the input terminal for the signal at the oscillator frequency, thus load the resonant oscillator circuit and cause changes in frequency therein. This is true particularly of changes in the transistor capacitances which have a disruptive effect particularly at fairly high frequencies. Moreover, the impedances of the mixer stage transistors are also affected by changes in the input signal and the operating voltage in a manner which has back effect on the oscillator frequency, when the oscillator is connected directly to the mixing stage.